1. Field of the Invention
The invention relates to an apparatus and process for the complete breaking down and incineration of tires and other hard-to-dispose-of waste products. More particularly, the invention relates to an apparatus and the process utilizing that apparatus for reducing to a very small size waste items which are very difficult to burn, and then incinerating those small particles. More specifically, the invention relates to an apparatus which includes a unique furnace design wherein the incineration of small particles within the furnace proceeds to the point that the particles are essentially completely burned. A heat recovery component of the invention is utilized to recover energy released by the incineration of the small particles.
2. Description of the Prior Art
It is well known that the disposal of waste rubber and plastic products--particularly old tires--is a problem of considerable concern, and has been for a number of years. In spite of a variety of plans aimed at the resolution of the problem, there exists a vast store of such waste products throughout the world and there is no indication that the store is diminishing. For example, the figure typically cited for the number of old tires dumped or otherwise disposed of in the United States alone is in excess of 200 million per year. For various reasons, the systems and devices currently utilized in the reduction of hard-to-burn waste materials are simply not satisfactory, in spite of the fact that most of the key problems are known.
There are a number of characteristics of rubber and plastic products which make their disposal quite difficult. First of all, they are comprised of extremely stable compounds. Although this stability makes them desirable, it also means that the time required for natural decomposition can extend over many hundreds of years. Secondly, such products generally are bulky and so tend to take up a considerable storage volume. The combination of their stable nature and bulkiness make them a waste-handling nightmare; they occupy a tremendous amount of space for a very long period of time. Another unfortunate characteristic of rubber and plastic products is their ignition-resistance--in part, because of the difficulty in breaking down the long-chain polymers--and the fact that when burned, their offgassing products are offensive and, in varying degrees, hazardous. However, these petroleum-based products have significant energy potential, and their burning, once initiated, continues for a long period of time--a serious threat in view of the vast piles of such waste products. These characteristics create further well-known problems in that when they are burned in bulk, or when they burn unexpectedly, they are environmental hazards of considerable public concern. This is readily apparent when one observes the attention focussed on any of the "mountains" of tires located throughout the country when it begins to burn.
One means utilized in the disposal of rubber tires is to chop or grind the tires into small pieces and use them as recyclable fillers in other products, such as blacktop for road surfaces. This method of disposal eliminates the environmental problems associated with waste burning; however, the volume of waste tires and other polymeric products far exceeds the volume of filler needed, and the gap is likely to widen. Therefore, alternative methods of disposal are still required if the goal is to cut down on the total volume of stored waste.
Currently, the most viable alternative to the recycling process described above--an alternative which is fairly well-known--is controlled burning of waste materials, by incineration or pyrolysis, wherein gas-cleaning equipment is used to overcome the offgassing problem. Incineration involves the oxidation of the material in an excess of air, while pyrolysis involves distillation of the material rather than combustion. Incineration provides a more complete break down of the waste in a shorter period of time, and unlike pyrolysis, it does not require an external fuel source to sustain the break down process. Specifically, incineration of waste tires and the like generally occurs at temperatures of about 1700.degree. F.--well above their ignition temperature--while the same products are pyrolyzed at about 600.degree. F. As a result, the by-products of incineration are primarily gases and unburned particulate matter, while the by-products of pyrolysis are a combination of gases, hydrocarbon liquids and particulate. The high temperature incineration products are immediately used to produce heat or electricity, while on the other hand, the products of pyrolysis--hydrocarbon gases and liquids--are often stored and used to produce energy at a later time. The two burning processes reduce waste material, but they do so in different ways and they result in essentially different products. The selection of one process over the other is generally determined by the product one wishes to obtain from the waste materials. It should be noted, however, that in most prior art incineration devices, high-temperature incineration naturally produces more undesirable nitrogen oxides than does the pyrolysis method. Therefore, if the goal is simply to reduce the volume of waste rather than create a particular by-product, the nitrogen oxide problem must be considered.
Kutrieb (U.S. Pat. No. 4,507,174 issued Mar. 26, 1985) and Roy (U.S. Pat. No. 4,740,270 issued Apr. 26, 1988) both describe pyrolysis devices for the reduction of waste tires to storable hydrocarbon by-products, wherein the by-products may be utilized to produce energy. It is readily apparent from these disclosures that a considerable amount of support equipment is required to make pyrolysis viable. Such equipment includes, but is not limited to, complex reaction chambers, compressors, storage means, and gas scrubbing means. There exists then a problem with such devices in that they require a considerable amount of energy to be operated and so they are generally not energy efficient. For this reason, such devices are of little practical usefulness; they comprise at present a very uneconomical means for disposing of the waste materials in question.
Incineration, on the other hand, generally requires less supplemental equipment, primarily because the resultant high-temperature products of combustion-carbon dioxide, gaseous water and traces of hydrocarbons and nitrous oxides--are used immediately to produce energy. Typically, this is achieved by linking an incineration furnace to a boiler and creating hot water or steam. For that reason, compressors and storage means are not required. A device for the incineration of waste rubber and plastic products is described by Eriksson et al. (U.S. Pat. No. 4,469,034 issued Sept. 4, 1984). Eriksson emphasizes the problem of nitrous oxide emissions associated with the incineration of such waste materials, and indicates that the utilization of small particles increases the incineration efficiency and reduces the amount of excess air required to oxidize the waste material. The reduction in the amount of air required leads to a reduction in the volume of nitrous oxides emitted by the device. Although burning of small particles is a well-known disposal method, as illustrated by the fluidized-bed combustion device described by Habib (U.S. Pat. No. 4,588,477), Eriksson discloses a cyclone furnace wherein the particles swirl within the furnace in a downward spiraling motion for an extended period of time, and the amount of air used to carry out the combustion process is regulated.
The Eriksson process and apparatus improve upon other prior art devices in that small particles burn more easily than bulk products. In addition, the reduction in the amount of combustion air introduced to the furnace minimizes the output of nitrous oxides. However, the heat transfer method utilized by Eriksson is not as efficient as it could be. Specifically, the heat needed to ignite the particles is radiated to those particles from the walls of the furnace which are actually some distance away from the particles themselves. Although the particular technique of spiraling the particles as described by Eriksson is useful, radiating heat through air to those particles is less efficient than conducting heat to the particles, principally because the air has a relatively low heat capacity. Another problem associated with the Eriksson process and apparatus is that larger particles of rubber waste are not caught in the swirling air; instead, they drop right to the bottom of the furnace and are then carried along with the combustion gases to a boiler connected to the furnace. It is anticipated that within the boiler essentially all of the remaining particles will eventually burn completely. Still another problem with the Eriksson device relates to the design of the cyclone furnace itself. Specifically, the furnace has "dead spots" in which the flue gas (the resultant product of the incineration of the waste particles) can dwell for extended periods of time without performing any useful function. These dead spots are in the corners of the furnace, and there may be a dead spot at the center of the spiraling path of particles. Finally, Eriksson states that in order to completely burn 90% of the rubber particles within the device described, those particles must be no greater than 1.5-2 millimeters in size. This size limitation means that an additional step is required to ensure that the particles can be completely burned. All of these problems lead to inefficient consumption of the waste particle fuel source.
In spite of these problems, Eriksson highlights the importance of burning waste materials in particle form rather than in bulk. Also, Eriksson shows that regulation of the air input, and burning of the particles at temperatures just above the combustion temperature of the waste material, minimizes the formation of hazardous offgases such as nitrous oxides. Nevertheless, as discussed above, the device described by Eriksson has inefficiencies which in all likelihood make it economically unacceptable. Therefore, in order to provide a means of burning waste materials in an economically efficient manner, what is needed is a process and apparatus wherein the means utilized to ignite waste particles has a greater heat transfer capacity than hot air. Also, what is needed is a process and apparatus wherein all waste particles proceed through incineration in the same manner and wherein all particles are burned completely within the furnace, regardless of any variations in size--that is, within certain reasonable limits. Further, what is needed is a process and apparatus wherein the furnace is designed such that there are no dead spots in which the resultant gases of incineration dwell within the furnace. Still further, what is needed is a process and apparatus wherein the size of the waste particles to be burned can vary significantly--from powder size to crumb size. Finally, what is needed is a completely integrated system wherein hard-to-dispose-of waste products may be broken down to particle size and the particles completely burned to yield a high-temperature combustion product the heat of which is recoverable by energy recapture means.